The invention relates to an injection-moulding method in which plastic material under pressure is injected from an antechamber which can be shut off by a shut-off means, into a mould cavity after opening of said shut-off means, and fills the mould cavity under pressure.
U.S. Pat. No. 2,952,041 discloses a method in which the pressure in the antechamber is due to the continuous rotation of an extrusion screw which continuously supplies molten plastic to the antechamber. The maximum pressure that can be achieved in this way is approximately 10.000 p.s.i. The expansion of the molten plastic material taking place after the opening of an orifice of the antechamber would per se not entirely fill the mould cavity. Rather, the filling is caused in part by the continuous addition of new material to the antechamber. The method disclosed in U.S. Pat. No. 2,052,041 eliminates the need for a ram extruder in injection moulding.
Unlike U.S. Pat. No. 2,052,041 the invention is less concerned with a rapid sequence of injection events but rather it is an object of the invention to decrease the time for the individual filling of the mould cavity.
Particularly in the production of small and thin-walled components it is essential for the injection operation to be terminated rapidly as otherwise partial hardening of the plastic material already occurs in the cooled mould during the injection procedure. In order to achieve high injection speeds, hitherto the advance speed of the screw or a separate injection piston which is possibly provided has been increased more and more. The invention is based on the thought that further development along those lines is not meaningful as, at high piston advance speeds, the molten material in the antechamber is primarily compacted, whereas that achieves little in terms of filling the mould cavity.
Therefore an object of the invention is to rapidly fill the mould cavity, particularly when injection-moulding thin and small mouldings, whereby the speed of the injection piston is to play no part or only a subordinate part.
In a previous application (cf. WO 01/03906) applicants have achieved this object on principle by compressing the molten material in the antechamber by means of a piston whereby the pressure obtained is sufficient to ensure that the subsequent expansion of the material enclosed in the antechamber is sufficient to fill the mould cavity.
Insofar as hitherto a pressure which is comparable to the pressure in the mould interior had been built up upstream of the shut-off means, prior to opening of the shut-off means, the only result of that pressure was that the mould was initially partially filled by expansion of the plastic material in the antechamber until after some delay the action of the screw advance movement came into effect. In contrast, WO 01/03906 is based on the notion that the entire mould cavity is filled merely by expansion of the supply of plastic material which has accumulated in the antechamber and which is under high pressure. If additionally a movement of the screw or other injection piston takes place intentionally or unintentionally, that only results in a modification of the adiabatic expansion method which in itself governs the filling of the mould. In practical terms that means that the pressure in the antechamber, which conventionally does not exceed 800 bars, is typically increased to over 1500 bars when carrying the invention into effect, and in particular that the volume of the antechamber is not reduced to a very substantial extent, as is usual, during the injection operation, but is entirely or predominantly maintained.
Practical experience with the ideas forwarded in WO 01/03906 showed that the weight of the articles produced under seemingly identical conditions and, therefore, the quality of the articles showed unexpected fluctuations. A possible explanation was sought in variations of the flow characteristics of the melt or in a deterioration of the shut-off valve. Finally, an essential cause for the less than perfect function of the known device was identified: there may be a difference between the volume of the antechamber at the beginning of the axial movement of the piston and the volume of the molten plastic encluded in the antechamber. This is due to the fact that the back-flow closure means provided at the tip of the plastifier is activated only by the axial movement of the screw. The moment at which the screw starts to act as a piston rather than a plastifier is, therefore, not well-defined. Therefore, the mass of material compressed by the piston varies between different injection cycles. This is the starting point for the invention which provides that the position of the piston and the pressure in the antechamber are monitored after the start of the axial movement of the piston and that the position of the piston, at which the closure means between antechamber and mould cavity is opened, is calculated taking into account at least one value of the measured pressure and of the position of the piston at which the measurement was performed.
The invention is based on the idea that starting with different masses of plastic material enclosed in the antechamber articles with identical weight and quality can be obtained if a pressure is chosen in the antechamber which compensates for the difference in mass. In detail this will be explained in connection with the drawings.